As is known to those skilled in the art, initial oscillation of a circuit after startup occurs at the first reached frequency at which the gain is greater than 1 and the signal being fed back is in-phase with the incoming signal. Circuits which have multiple possibilities of steady state oscillation will oscillate at the frequency which is first reached, oscillate at several of the frequencies, or start at one frequency and then switch to another frequency depending upon the interaction of two or more active devices exhibiting non-linear behavior within the circuit and/or the characteristics of the remaining portions of the circuit. Multiple frequency oscillation is undesirable where a stable reference source is the main design objective. It is therefore a design objective that the circuit have the possibility of oscillation at only one frequency even with changes in environmental conditions and power supply perturbances.
In the past, inductors have often been used to effectively lower the gain at the lower frequencies to less than 1 so that oscillations may occur only at the desired high frequency. However, inductors are expensive to make in integrated circuits because they require a large amount of chip area. It is thus desirable to design an oscillator which does not require inductors in the main signal path or the feedback path.
The present invention accomplishes the desired results of eliminating inductors by using an odd number of stages whereby the phase of the feedback and input signals are of substantially opposite phases at DC or near DC frequencies and converge to being in-phase at the desired high frequency. This is accomplished by placing capacitor-resistor combinations intermediate the stages, such that the phase shift for a given stage where there are three total stages is 120.degree. at the desired oscillatory frequency. The phase shift networks increase inphase shift with increasing frequencies from DC to the desired frequency, while the phase shift of the amplifying FET decreases from substantially 180.degree. at DC to in the neighborhood of 90.degree. at the desired frequency. As is known, phase shift of an FET, with increasing frequencies of operation, decreases from initially 180.degree. to in the neighborhood of 90.degree. at signal frequencies in the L band. On the other hand, a phase shifting network of the type shown, has zero degrees phase shift at low frequencies and approximately 30.degree. phase shift at the frequency of the L band. Thus, the inner stage phase shift networks are complimentary to the phase shift action of the individual amplifying or active portions of each stage to keep the total amplifier phase shift more nearly constant. As is also known to those skilled in the art, the phase shift of the FET is caused by internal parasitic (undesirable) capacitances. These parasitic capacitances have a value dependent in part upon the bias voltage applied to the gate of the FET. If a variable voltage is thus applied to one or more stages of the oscillator, a variable frequency oscillator can be obtained.
It is an object of the present invention to provide an improved oscillator circuit which eliminates the requirements for inductors and which can be easily applied to integrated circuit technology.